Distortion in an audio system may occur in various forms including analog and/or digital distortion in amplifiers and signal processors. Analog distortion may include harmonic distortion in amplifiers and loudspeakers, intermodulation distortion in amplifiers and loudspeakers, and crossover distortion in push-pull amplifiers.
One form of analog distortion that is difficult to eradicate is harmonic distortion. Harmonic distortion occurs in amplifiers, signal processors and loudspeakers. Many scientific studies have sought to isolate the causes of harmonic distortion and some causes have been identified including non-linearity in transfer function(s) associated with one or more parts of a system. For example, harmonic distortion in a loudspeaker driver may include distortion due to mechanical non-linearity of an associated diaphragm suspension, hysteresis in an associated magnetic circuit, back emf associated with a voice coil and/or a voice coil that operates outside of its linear excursion range.
However, it has also been assumed that a substantially distortion-free amplifier driving a substantially distortion-free loudspeaker driver should result in little or no distortion. Accordingly, most research effort has been applied to locating and correcting distortion in amplifiers and loudspeaker drivers. It has not been appreciated that the assumption may be incorrect and/or that a further cause of distortion in a loudspeaker system may be due to interaction between the driver of the loudspeaker system and its enclosure including non-linear compression of air both inside the loudspeaker enclosure and outside the enclosure including around an area of radiation. Non-linear compression of air may generate mostly Second-order harmonic distortion components in the acoustic output of a loudspeaker system. The latter distortion components may also increase with sound pressure level (SPL).
Moreover, harmonic distortion components caused by non-linear compression of air is in addition to harmonic distortion components arising from an amplifier, a loudspeaker driver and/or other components of a loudspeaker system. As noted above, a loudspeaker system with substantially distortion-free components may still generate distortion. Solutions attempted in the prior art to address this problem include use of motional feedback, pre-distortion compensation and pre-distortion compensation with feedback. However, motional feedback cannot correct distortion that is generated in an audio signal path beyond the cone of a loudspeaker driver, while analog pre-distortion compensation cannot store enough data to predict the distortion sufficiently to fully compensate or cancel the distortion. Digital pre-distortion compensation may be subject to digital distortion and may also require extremely fast processing. Pre-distortion also has a disadvantage in that it has to be matched to a loudspeaker system.
The present invention may be adapted to manage and/or at least reduce Second-order and/or Third-order components of harmonic distortion including distortion components arising from non-linear compression of air and/or other causes without using feedback and without being matched to a loudspeaker system.
A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge in Australia or elsewhere as at the priority date of any of the disclosures or claims herein. Such discussion of prior art in this specification is included to explain the context of the present invention in terms of the inventor's knowledge and experience.
Throughout the description and claims of this specification the words “comprise” or “include” and variations of those words, such as “comprises”, “includes”, “comprising” or “including”, are not intended to exclude other additives, components, integers or steps.